Many signals have oscillatory characteristics, ranging from a simple sine wave to noisy signals which may vary greatly in frequency and amplitude over time. Biological signals often display oscillatory characteristics and are generally prone to noise, either inherent in the signal and/or in the measuring apparatus and some are also liable to large and rapid regular or irregular fluctuations in their frequency and amplitude.
Thus, measurement of signals like biological signals often gives rise to false positive or negative detection of the measured characteristic when traditional methods of detection based on constant thresholds are used. Such false detection's lead to inaccuracies in the measurement of the characteristics of the signal.
One area where inaccuracies in the measurement of the signal may be particularly onerous is in the detection of heart beats. Heart beats are typically monitored in hospitals or the like where it is necessary to monitor the functions of the heart for diagnosis purposes or to give an indication when a patient requires medical attention. However, heart rate is also often monitored to assist in the evaluation of the performance of athletes, to monitor stress levels of individuals in certain situations and have been used to give an indication of whether someone is being untruthful. However, the most critical detection process is usually monitoring a patient in a care situation.
Heart beats may be detected by monitoring the pulsatile blood pressure signal. However, the blood pressure signal reflects the fact that rapid changes in heart rate can occur within a single beat, which can give rise to false positive or false negative detection of a heart beat. For example, in the case of pulse arrhythmia, there is varying (often regularly) beat-to-beat interval and amplitude, which need not be rejected during the detection. In addition, depending on the measurement site, the pulse wave often contains significant amounts of wave reflection, which must be accommodated for in order to accurately detect each heart beat and not to be mistaken for the irregular variations of frequency and amplitude during arrhythmia.
Problems similar to distinguishing between variations of the blood pressure signal over time due to arrhythmia, reflections and changes in amplitude, frequency and baseline are often present in biological signals and in other oscillatory signals which exhibit highly variable and or noisy characteristics. These variations cause significant difficulties in accurately measuring the characteristics of the signal such as frequency and amplitude.